Abstract
Flavins are long-known redox enzyme cofactors, discovered early in the history of enzymology to play essential roles in a whole range of enzymes : electron transfer enzymes, oxidases, monooxygenases... These molecules all have an isoalloxazine core and vary according to the substituents present on the ring. For a long time, this family was essentially limited to three families : riboflavin, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). The lectures 5 and 6 describe recent discoveries showing a much greater variety of these flavins, thanks in particular to the possibility of introducing substituents on the N5 position of the isoalloxazine ring. For example, a flavin N5-oxide is present in many bacterial enzymes catalyzing oxidation (in the biosynthesis of the antibiotic enterocin, in the degradation of uracil or the degradation of the environmental toxicant hexachlorobenzene, etc.). In addition, a new natural decarboxylation chemistry has been demonstrated, involving the intervention of a prenylated flavin in the active site of these novel decarboxylases, as in the case of the biosynthesis of ubiquinone, vitamin Q. Finally, a flavin bearing a methylene group on the N5 position is the methylating agent in a whole series of new methylation reactions, catalyzed by original methylases (transfer RNA methylation, for example). The biosynthesis of these new flavins and their chemical reactivity are discussed.
